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FEATURES Computes: True RMS Value Average Rectified Value Absolute Value Provides: 200 mV Full-Scale Input Range (Larger Inputs with Input Attenuator) High Input Impedance of 1012 V Low Input Bias Current: 25 pA Max High Accuracy: 0.3 mV 0.3% of Reading RMS Conversion with Signal Crest Factors Up to 5 Wide Power Supply Range: +2.8 V, -3.2 V to 16.5 V Low Power: 200 mA Max Supply Current Buffered Voltage Output No External Trims Needed for Specified Accuracy AD737--An Unbuffered Voltage Output Version with Chip Power Down also Available
Low Cost, Low Power, True RMS-to-DC Converter AD736
FUNCTIONAL BLOCK DIAGRAM
8k CC 1 FULL WAVE RECTIFIER INPUT AMPLIFIER rms CORE OUTPUT AMPLIFIER
5 CAV
AD736
8 COM
VIN 2
8k
7 +VS
CF 3
BIAS SECTION
6 OUTPUT
-VS 4
GENERAL DESCRIPTION
The AD736 is a low power, precision, monolithic true rms-to-dc converter. It is laser trimmed to provide a maximum error of 0.3 mV 0.3% of reading with sine wave inputs. Furthermore, it maintains high accuracy while measuring a wide range of input waveforms, including variable duty cycle pulses and triac (phase) controlled sine waves. The low cost and small physical size of this converter make it suitable for upgrading the performance of non-rms precision rectifiers in many applications. Compared to these circuits, the AD736 offers higher accuracy at equal or lower cost. The AD736 can compute the rms value of both ac and dc input voltages. It can also be operated ac-coupled by adding one external capacitor. In this mode, the AD736 can resolve input signal levels of 100 V rms or less, despite variations in temperature or supply voltage. High accuracy is also maintained for input waveforms with crest factors of 1 to 3. In addition, crest factors as high as 5 can be measured (while introducing only 2.5% additional error) at the 200 mV full-scale input level. The AD736 has its own output buffer amplifier, thereby providing a great deal of design flexibility. Requiring only 200 A of power supply current, the AD736 is optimized for use in portable multimeters and other battery-powered applications. The AD736 allows the choice of two signal input terminals: a high impedance FET input (1012 W) that directly interfaces with high Z input attenuators and a low impedance input (8 kW)
that allows the measurement of 300 mV input levels while operating from the minimum power supply voltage of +2.8 V, -3.2 V. The two inputs may be used either single-ended or differentially. The AD736 has a 1% reading error bandwidth that exceeds 10 kHz for the input amplitudes from 20 mV rms to 200 mV rms while consuming only 1 mW. The AD736 is available in four performance grades. The AD736J and AD736K grades are rated over the commercial temperature ranges of 0C to +70C and -20C to +85C. The AD736A and AD736B grades are rated over the industrial temperature range of -40C to +85C. The AD736 is available in three low cost, 8-lead packages: PDIP, SOIC, and CERDIP.
PRODUCT HIGHLIGHTS
1. The AD736 is capable of computing the average rectified value, absolute value, or true rms value of various input signals. 2. Only one external component, an averaging capacitor, is required for the AD736 to perform true rms measurement. 3. The low power consumption of 1 mW makes the AD736 suitable for many battery-powered applications. 4. A high input impedance of 1012 W eliminates the need for an external buffer when interfacing with input attenuators. 5. A low impedance input is available for those applications requiring an up to 300 mV rms input signal operating from low power supply voltages.
REV. E
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective companies.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 www.analog.com Fax: 781/326-8703 (c) 2003 Analog Devices, Inc. All rights reserved.
AD736-SPECIFICATIONS
Parameter
TRANSFER FUNCTION CONVERSION ACCURACY Total Error, Internal Trim1 All Grades TMIN to TMAX A and B Grades J and K Grades vs. Supply Voltage @ 200 mV rms Input @ 200 mV rms Input DC Reversal Error, DC-Coupled Nonlinearity2, 0 mV-200 mV Total Error, External Trim
(@ 25 C 5 V supplies, ac-coupled with 1 kHz sine wave input applied, unless otherwise noted.)
AD736J/AD736A Min Typ Max VOUT = Avg VIN AD736K/AD736B Min Typ Max VOUT = Avg VIN
Conditions
Unit
()
2
()
2
1 kHz Sine Wave AC-Coupled Using CC 0 mV rms-200 mV rms 200 mV-1 V rms @ 200 mV rms @ 200 mV rms VS = 5 V to 16.5 V VS = 5 V to 3 V 0 0
0.3/0.3 -1.2 0.7/0.7 0.007 +0.06 -0.18 1.3 0.25 0.1/0.5 0.7 2.5
0.5/0.5 2.0
0.2/0.2 0.3/0.3 mV/% of Reading -1.2 2.0 % of Reading 0.5/0.5 mV/% of Reading % of Reading/C +0.1 -0.3 2.5 0.35 %/V %/V % of Reading % of Reading mV/% of Reading % Additional Error % Additional Error
0.007 +0.1 -0.3 2.5 0.35 0 0 +0.06 -0.18 1.3 0.25 0.1/0.3 0.7 2.5
@ 600 mV dc @ 100 mV rms 0 0 mV rms-200 mV rms
0
ERROR VERSUS CREST FACTOR3 Crest Factor 1 to 3 CAV, CF = 100 F Crest Factor = 5 CAV, CF = 100 F INPUT CHARACTERISTICS High Impedance Input (Pin 2) Signal Range Continuous rms Level Continuous rms Level Peak Transient Input Peak Transient Input Peak Transient Input Input Resistance Input Bias Current Low Impedance Input (Pin 1) Signal Range Continuous rms Level Continuous rms Level Peak Transient Input Peak Transient Input Peak Transient Input Input Resistance Maximum Continuous Nondestructive Input Input Offset Voltage4 J and K Grades A and B Grades vs. Temperature vs. Supply vs. Supply
VS = +2.8 V, -3.2 V VS = 5 V to 16.5 V VS = +2.8 V, -3.2 V VS = 5 V VS = 16.5 V VS = 3 V to 16.5 V
200 1 0.9 2.7 4.0 1012 1 25 4.0 1012 1 0.9 2.7
200 1
25
mV rms V rms V V V W pA
VS = +2.8 V, -3.2 V VS = 5 V to 16.5 V VS = +2.8 V, -3.2 V VS = 5 V VS = 16.5 V 6.4 All Supply Voltages AC-Coupled
300 l 1.7 3.8 11 8 1.7 3.8 11 8
300 l
9.6 12 3 3 30 150
6.4
9.6 12 3 3 30 150
mV rms V rms V V V kW V p-p mV mV V/C V/V V/V
VS = 5 V to 16.5 V VS = 5 V to 3 V
8 50 80
8 50 80
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AD736
Parameter
OUTPUT CHARACTERISTICS Output Offset Voltage J and K Grades A and B Grades vs.Temperature vs. Supply Output Voltage Swing 2 kW Load 2 kW Load 2 kW Load No Load Output Current Short-Circuit Current Output Resistance FREQUENCY RESPONSE High Impedance Input (Pin 2) For 1% Additional Error VIN = 1 mV rms VIN = 10 mV rms VIN = 100 mV rms VIN = 200 mV rms 3 dB Bandwidth VIN = 1 mV rms VIN = 10 mV rms VIN = 100 mV rms VIN = 200 mV rms FREQUENCY RESPONSE Low Impedance Input (Pin 1) For 1% Additional Error VIN = 1 mV rms VIN = 10 mV rms VIN = 100 mV rms VIN = 200 mV rms 3 dB Bandwidth VIN = l mV rms VIN = 10 mV rms VIN = 100 mV rms VIN = 200 mV rms POWER SUPPLY Operating Voltage Range Quiescent Current 200 mV rms, No Load TEMPERATURE RANGE Operating, Rated Performance Extended Commercial (-20C to +85C) Commercial (0C to 70C) Industrial (-40C to +85C)
Conditions
AD736J/AD736A Min Typ Max
AD736K/AD736B Min Typ Max
Unit
0.1 1 50 50 0 to 1.6 0 to 3.6 0 to 4 0 to 4 2 1.7 3.8 5 12 3 0.2
VS = 5 V to 16.5 V VS = 5 V to 3 V VS = +2.8 V, -3.2 V VS = 5 V VS = 16.5 V VS = 16.5 V
0.5 0.5 20 130
0.1 1 50 50 0 to 1.6 0 to 3.6 0 to 4 0 to 4 2 1.7 3.8 5 12 3 0.2
0.3 0.3 20 130
mV mV V/C V/V V/V V V V V mA mA W
@ dc
Sine Wave Input 1 6 37 33 Sine Wave Input 5 55 170 190 5 55 170 190 kHz kHz kHz kHz 1 6 37 33 kHz kHz kHz kHz
Sine Wave Input 1 6 90 90 Sine Wave Input 5 55 350 460 +2.8, -3.2 5 160 230 16.5 200 270 5 55 350 460 +2.8, -3.2 5 160 230 16.5 200 270 kHz kHz kHz kHz V A A 1 6 90 90 kHz kHz kHz kHz
Zero Signal Sine Wave Input
AD736JR AD736JN AD736A
AD736KR AD736KN AD736B
NOTES l Accuracy is specified with the AD736 connected as shown in Figure 1 with capacitor C C. 2 Nonlinearity is defined as the maximum deviation (in percent error) from a straight line connecting the readings at 0 mV rms and 200 mV rms. Output offset voltage is adjusted to zero. 3 Error versus crest factor is specified as additional error for a 200 mV rms signal. Crest factor = V PEAK/V rms. 4 DC offset does not limit ac resolution. Specifications are subject to change without notice. Specifications shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels.
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AD736
ABSOLUTE MAXIMUM RATINGS 1 PIN CONFIGURATION 8-Lead PDIP (N-8), 8-Lead SOIC (RN-8), 8-Lead CERDIP (Q-8)
8k CC 1 FULL WAVE RECTIFIER INPUT AMPLIFIER rms CORE OUTPUT AMPLIFIER
5 CAV
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.5 V Internal Power Dissipation2 . . . . . . . . . . . . . . . . . . . . 200 mW Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VS Output Short-Circuit Duration . . . . . . . . . . . . . . . . Indefinite Differential Input Voltage . . . . . . . . . . . . . . . . . . +VS and -VS Storage Temperature Range (Q) . . . . . . . . . -65C to +150C Storage Temperature Range (N, R) . . . . . . . -65C to +125C Lead Temperature Range (Soldering 60 sec) . . . . . . . . . 300C ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 V
NOTES 1 Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. 2 8-Lead PDIP Package: JA = 165C/W 8-Lead CERDIP Package: JA = 110C/W 8-Lead SOIC Package: JA = 155C/W
AD736
8 COM
VIN 2
8k
7 +VS
CF 3
BIAS SECTION
6 OUTPUT
-VS 4
ORDERING GUIDE
Model AD736JN AD736KN AD736JR AD736KR AD736AQ AD736BQ AD736JR-Reel AD736JR-Reel-7 AD736KR-Reel AD736KR-Reel-7
Temperature Range 0C to +70C 0C to +70C -20C to +85C -20C to +85C -40C to +85C -40C to +85C -20C to +85C -20C to +85C -20C to +85C -20C to +85C
Package Description PDIP PDIP SOIC SOIC CERDIP CERDIP SOIC SOIC SOIC SOIC
Package Option N-8 N-8 R-8 R-8 Q-8 Q-8 R-8 R-8 R-8 R-8
CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD736 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
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Typical Performance Characteristics-AD736
0.7
16
PEAK INPUT BEFORE CLIPPING - V
VIN = 200mV rms 1kHz SINE WAVE CAV = 100 F CF = 22 F
10mV
ADDITIONAL ERROR - % OF READING
DC-COUPLED 14 12
INPUT LEVEL - rms
0.5
VIN = 1kHz SINE WAVE INPUT AC-COUPLED VS = 5V 1mV
0.3
10 PIN 1 8 PIN 2 6 4 2 0 0 2 4 6 8 10 12 SUPPLY VOLTAGE - V 14 16
0.1 0 -0.1
100 V
-0.3
-0.5
0
2
4 6 8 10 12 SUPPLY VOLTAGE - V
14
16
10 V 100
1k 10k -3dB FREQUENCY - Hz
100k
TPC 1. Additional Error vs. Supply Voltage
TPC 2. Maximum Input Level vs. Supply Voltage
TPC 3. Peak Buffer Output vs. Supply Voltage
10V SINE WAVE INPUT, VS = 5V, CAV = 22 F, CF = 4.7 F, CC = 22 F 1V
10V
ADDITIONAL ERROR - % OF READING
SINE WAVE INPUT, VS = 5V, CAV = 22 F, CF = 4.7 F, CC = 22 F 1V INPUT LEVEL - rms
6 3ms BURST OF 1kHz = 3 CYCLES CAV = 10 F 200mV rms SIGNAL VS = 5V CC = 22 F CAV = 33 F CF = 100 F
5
INPUT LEVEL - rms
4
100mV 1% ERROR 10mV -3dB 1mV 10% ERROR 100 V 0.1
100mV 1% ERROR 10mV
3
2
1mV
10% ERROR -3dB
1
CAV = 100 F CAV = 250 F
1
10 100 FREQUENCY - kHz
1000
100 V 0.1
1
10 100 FREQUENCY - kHz
1000
0
1
2 3 4 CREST FACTOR - V PEAK / V rms
5
TPC 4. Frequency Response Driving Pin 1
TPC 5. Frequency Response Driving Pin 2
TPC 6. Additional Error vs. Crest Factor vs. CAV
0.8
ADDITIONAL ERROR - % OF READING
600 VIN = 200mV rms 1kHz SINE WAVE CAV = 100 F CF = 22 F VIN = 1kHz SINE WAVE INPUT VS = 5V CAV = 22 F CC = 10 F
10mV VIN = 1kHz SINE WAVE INPUT AC-COUPLED VS = 5V INPUT LEVEL - rms
1.0
0.6 0.4 0.2 0 -0.2 -0.4 -0.6
DC SUPPLY CURRENT - A
500
1mV
400
300
100 V
200
-0.8 -60 -40 -20
0 20 40 60 80 100 120 140 TEMPERATURE - C
100
0
0.2
0.4 0.6 0.8 rms INPUT LEVEL - V
10 V 100
1k 10k -3dB FREQUENCY - Hz
100k
TPC 7. Additional Error vs. Temperature
TPC 8. DC Supply Current vs. rms lnput Level
TPC 9. -3 dB Frequency vs. rms Input Level (Pin 2)
REV. E
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AD736
1.0 0.5
100 VIN = 200mV rms CC = 47 F CF = 47 F VS = 5V
1V
ERROR - % OF READING
-1%
INPUT LEVEL - rms
-0.5%
0 -0.5 -1.0 -1.5 -2.0 VIN = SINE WAVE @ 1kHz CAV = 22 F, CC = 47 F, CF = 4.7 F, VS = 5V 100mV INPUT LEVEL - rms 1V 2V
100mV
CAV - F
10 -0.5%
10mV VIN = SINE WAVE AC-COUPLED CAV = 10 F, CC = 47 F, CF = 47 F, VS = 5V
-1% 1 10
-2.5 10mV
100 FREQUENCY - Hz
1k
1mV
1
10 100 FREQUENCY - Hz
1k
TPC 10. Error vs. rms Input Voltage (Pin 2), Output Buffer Offset Is Adjusted to Zero
TPC 11. CAV vs. Frequency for Specified Averaging Error
TPC 12. rms Input Level vs. Frequency for Specified Averaging Error
4.0
1V VS = 5V CC = 22 F CF = 0 F
10nA
INPUT BIAS CURRENT - pA
3.5 INPUT LEVEL - rms
100mV CAV = 10 F 10mV CAV = 33 F
1nA
INPUT BIAS CURRENT
3.0
CAV = 100 F
100pA
2.5
10pA
2.0
1mV
1.5
1pA
1.0
0
2
4 6 8 10 12 SUPPLY VOLTAGE - V
14
16
100 V 1ms
10ms
100ms 1s SETTLING TIME
10s
100s
100fA -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE - C
TPC 13. Pin 2 Input Bias Current vs. Supply Voltage
TPC 14. Settling Time vs. rms Input Level for Various Values of CAV
TPC 15. Pin 2 Input Bias Current vs. Temperature
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REV. E
AD736
CALCULATING SETTLING TIME USING TPC 14
TPC 14 may be used to closely approximate the time required for the AD736 to settle when its input level is reduced in amplitude. The net time required for the rms converter to settle is the difference between two times extracted from the graph--the initial time minus the final settling time. As an example, consider the following conditions: a 33 F averaging capacitor, an initial rms input level of 100 mV, and a final (reduced) input level of 1 mV. From TPC 14, the initial settling time (where the 100 mV line intersects the 33 F line) is approximately 80 ms. The settling time corresponding to the new or final input level of 1 mV is approximately 8 seconds. Therefore, the net time for the circuit to settle to its new value is 8 seconds minus 80 ms, which is 7.92 seconds. Note that because of the smooth decay characteristic inherent with a capacitor/diode combination, this is the total settling time to the final value (i.e., not the settling time to 1%, 0.1%, and so on, of the final value). Also, this graph provides the worst-case settling time, since the AD736 settles very quickly with increasing input levels.
TYPES OF AC MEASUREMENT
the waveform being measured. For example, the average absolute value of a sine wave voltage is 0.636 that of VPEAK; the corresponding rms value is 0.707 times VPEAK. Therefore, for sine wave voltages, the required scale factor is 1.11 (0.707 divided by 0.636). In contrast to measuring the average value, true rms measurement is a universal language among waveforms, allowing the magnitudes of all types of voltage (or current) waveforms to be compared to one another and to dc. RMS is a direct measure of the power or heating value of an ac voltage compared to that of a dc voltage; an ac signal of 1 V rms produces the same amount of heat in a resistor as a 1 V dc signal. Mathematically, the rms value of a voltage is defined (using a simplified equation) as
V rms =
Avg V 2
()
The AD736 is capable of measuring ac signals by operating as either an average responding or a true rms-to-dc converter. As its name implies, an average responding converter computes the average absolute value of an ac (or ac and dc) voltage or current by full wave rectifying and low-pass filtering the input signal; this approximates the average. The resulting output, a dc average level, is then scaled by adding (or reducing) gain; this scale factor converts the dc average reading to an rms equivalent value for
This involves squaring the signal, taking the average, and then obtaining the square root. True rms converters are smart rectifiers; they provide an accurate rms reading regardless of the type of waveform being measured. However, average responding converters can exhibit very high errors when their input signals deviate from their precalibrated waveform; the magnitude of the error depends on the type of waveform being measured. As an example, if an average responding converter is calibrated to measure the rms value of sine wave voltages and then is used to measure either symmetrical square waves or dc voltages, the converter will have a computational error 11% (of reading) higher than the true rms value (see Table I).
Table I. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms
Waveform Type 1 V Peak Amplitude Undistorted Sine Wave Symmetrical Square Wave Undistorted Triangle Wave Gaussian Noise (98% of Peaks <1 V) Rectangular Pulse Train SCR Waveforms 50% Duty Cycle 25% Duty Cycle
Crest Factor (VPEAK/V rms) 1.414 1.00 1.73 3 2 10 2 4.7
True rms Value 0.707 V 1.00 V 0.577 V 0.333 V 0.5 V 0.1 V 0.495 V 0.212 V
Average Responding Circuit Calibrated to Read rms Value of Sine Waves Will Read 0.707 V 1.11 V 0.555 V 0.295 V 0.278 V 0.011 V 0.354 V 0.150 V
% of Reading Error Using Average Responding Circuit 0 11.0 -3.8 -11.4 -44 -89 -28 -30
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AD736
AD736 THEORY OF OPERATION
As shown by Figure 1, the AD736 has five functional subsections: the input amplifier, full-wave rectifier, rms core, output amplifier, and bias section. The FET input amplifier allows both a high impedance, buffered input (Pin 2) and a low impedance, widedynamic-range input (Pin 1). The high impedance input, with its low input bias current, is well suited for use with high impedance input attenuators. The output of the input amplifier drives a full-wave precision rectifier, which in turn, drives the rms core. It is in the core that the essential rms operations of squaring, averaging, and square rooting are performed, using an external averaging capacitor, CAV. Without CAV, the rectified input signal travels through the core unprocessed, as is done with the average responding connection (Figure 2). A final subsection, an output amplifier, buffers the output from the core and also allows optional low-pass filtering to be performed via the external capacitor, CF, connected across the feedback path of the amplifier. In the average responding connection, this is where all of the averaging is carried out. In the rms circuit, this additional filtering stage helps reduce any output ripple that was not removed by the averaging capacitor, CAV.
CC 10 F (OPTIONAL) +
RMS MEASUREMENT--CHOOSING THE OPTIMUM VALUE FOR CAV
Since the external averaging capacitor, CAV, holds the rectified input signal during rms computation, its value directly affects the accuracy of the rms measurement, especially at low frequencies. Furthermore, because the averaging capacitor appears across a diode in the rms core, the averaging time constant increases exponentially as the input signal is reduced. This means that as the input level decreases, errors due to nonideal averaging decrease while the time it takes for the circuit to settle to the new rms level increases. Therefore, lower input levels allow the circuit to perform better (due to increased averaging) but increase the waiting time between measurements. Obviously, when selecting CAV, a trade-off between computational accuracy and settling time is required.
CC 10 F + (OPTIONAL) CC VIN VIN
2
8k
1
AD736
FULL WAVE RECTIFIER INPUT AMPLIFIER BIAS SECTION rms CORE 8k
8
COM
+VS
7
+VS
CF
3
OUTPUT
6
VOUT
CURRENT MODE ABSOLUTE VALUE CC
1 8
-VS -VS
4
OUTPUT AMPLIFIER
5
CAV
COM
+ CF 33 F POSITIVE SUPPLY 0.1 F +VS
8k VIN
7 +VS
VIN
2
COMMON 0.1 F NEGATIVE SUPPLY -VS
FET OP AMP IB<10pA rms TRANSLINEAR CORE
8k
Figure 2. AD736 Average Responding Circuit
6
CF
3
RMS OUTPUT
RAPID SETTLING TIMES VIA THE AVERAGE RESPONDING CONNECTION
-VS
4
5
CAV
CAV 33 F + CF 10 F (OPTIONAL) + POSITIVE SUPPLY 0.1 F COMMON 0.1 F NEGATIVE SUPPLY -VS +VS
Because the average responding connection shown in Figure 2 does not use the CAV averaging capacitor, its settling time does not vary with input signal level; it is determined solely by the RC time constant of CF and the internal 8 kW resistor in the output amplifier's feedback path.
Figure 1. AD736 True rms Circuit
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REV. E
AD736
DC ERROR, OUTPUT RIPPLE, AND AVERAGING ERROR AC MEASUREMENT ACCURACY AND CREST FACTOR
Figure 3 shows the typical output waveform of the AD736 with a sine wave input applied. As with all real-world devices, the ideal output of VOUT = VIN is never exactly achieved; instead, the output contains both a dc and an ac error component.
EO IDEAL EO DC ERROR = EO - EO (IDEAL)
AVERAGE EO = EO DOUBLE-FREQUENCY RIPPLE TIME
The crest factor of the input waveform is often overlooked when determining the accuracy of an ac measurement. Crest factor is defined as the ratio of the peak signal amplitude to the rms amplitude (crest factor = VPEAK/V rms). Many common waveforms, such as sine and triangle waves, have relatively low crest factors (2). Other waveforms, such as low duty cycle pulse trains and SCR waveforms, have high crest factors. These types of waveforms require a long averaging time constant (to average out the long time periods between pulses). TPC 6 shows the additional error versus the crest factor of the AD736 for various values of CAV.
SELECTING PRACTICAL VALUES FOR INPUT COUPLING (CC), AVERAGING (CAV), AND FILTERING (CF) CAPACITORS
Figure 3. Output Waveform for Sine Wave Input Voltage
As shown, the dc error is the difference between the average of the output signal (when all the ripple in the output has been removed by external filtering) and the ideal dc output. The dc error component is therefore set solely by the value of the averaging capacitor used--no amount of post filtering (i.e., using a very large CF) will allow the output voltage to equal its ideal value. The ac error component, an output ripple, may be easily removed by using a large enough post filtering capacitor, CF. In most cases, the combined magnitudes of both the dc and ac error components need to be considered when selecting appropriate values for capacitors CAV and CF. This combined error, representing the maximum uncertainty of the measurement, is termed the averaging error and is equal to the peak value of the output ripple plus the dc error. As the input frequency increases, both error components decrease rapidly; if the input frequency doubles, the dc error and ripple reduce to one quarter and one half of their original values, respectively, and rapidly become insignificant.
Table II provides practical values of CAV and CF for several common applications. The input coupling capacitor, CC, in conjunction with the 8 k internal input scaling resistor, determines the -3 dB low frequency rolloff. This frequency, FL, is equal to FL = 1 2(8 , 000)(TheValue of CC in Farads)
Note that at FL, the amplitude error is approximately -30% (-3 dB) of reading. To reduce this error to 0.5% of reading, choose a value of CC that sets FL at one tenth of the lowest frequency to be measured. In addition, if the input voltage has more than 100 mV of dc offset, then the ac coupling network shown in Figure 6 should be used in addition to capacitor CC.
Table II. AD737 Capacitor Selection Chart
Application General-Purpose rms Computation
RMS Input Level 0 V-1 V 0 mV-200 mV
Low Frequency Cutoff (-3 dB) 20 Hz 200 Hz 20 Hz 200 Hz 20 Hz 200 Hz 20 Hz 200 Hz 50 Hz 60 Hz 50 Hz 60 Hz 300 Hz 20 Hz
Max Crest Factor 5 5 5 5
CAV 150 F 15 F 33 F 3.3 F None None None None 100 F 82 F 50 F 47 F 1.5 F 100 F
CF 10 F 1 F 10 F 1 F 33 F 3.3 F 33 F 3.3 F 33 F 27 F 33 F 27 F 0.5 F 68 F
Settling Time* to 1% 360 ms 36 ms 360 ms 36 ms 1.2 sec 120 ms 1.2 sec 120 ms 1.2 sec 1.0 sec 1.2 sec 1.0 sec 18 ms 2.4 sec
General-Purpose Average Responding SCR Waveform Measurement
0 V-1 V 0 mV-200 mV 0 mV-200 mV 0 mV-100 mV
5 5 5 5 3 10
Audio Applications Speech Music
0 mV-200 mV 0 mV-100 mV
*Settling time is specified over the stated rms input level with the input signal increasing from zero. Settling times are greater for decreasing amplitude input signals.
REV. E
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AD736 Application Circuits
OPTIONAL AC COUPLING CAPACITOR VIN 0.01 F 1kV 200mV 9M 2V 900k 20V 90k 200V 10k -VS -VS -VS
4
CC 10 F + (OPTIONAL) +VS CC 8k
1
AD736
FULL WAVE RECTIFIER INPUT AMPLIFIER
8
COM
1N4148
VIN
2
+VS 8k
7
47k 1W
1F OUTPUT
+VS
1N4148
CF
3
BIAS SECTION rms CORE
6
OUTPUT
OUTPUT AMPLIFIER
CAV
5
+ 1F CAV 33 F + CF 10 F (OPTIONAL)
Figure 4. AD736 with a High Impedance Input Attenuator
CC 10 F 6 +
-IN
3 2
AD711
CC
1
8k
AD736
FULL WAVE RECTIFIER INPUT AMPLIFIER
8
COM
VIN +IN INPUT IMPEDANCE: 1012 INPUT IMPEDANCE: 10pF -VS -VS
4 2
+VS 8k
7
1F OUTPUT
+VS
CF
3
BIAS SECTION rms CORE
6
OUTPUT
OUTPUT AMPLIFIER
CAV
5
+ 1F CAV 33 F + CF (OPTIONAL) 10 F
Figure 5. Differential Input Connection
-10-
REV. E
AD736
DC-COUPLED CC 10 F + AC-COUPLED CC 8k
1
(OPTIONAL) COM
AD736
FULL WAVE RECTIFIER INPUT AMPLIFIER
8
DC-COUPLED VIN 0.1 F AC-COUPLED 1M
VIN
2
8k
+VS
7
1F OUTPUT
CF
3
BIAS SECTION
6
-VS +VS 1M 39M OUTPUT VOS -VS ADJUST + 1F CAV 33 F +
4
OUTPUT AMPLIFIER CAV rms CORE
5
CF 10 F (OPTIONAL)
Figure 6. External Output VOS Adjustment
CF 10 F + CC
1
8k
AD736
FULL WAVE RECTIFIER INPUT AMPLIFIER
8
COM
0.1 F VIN 1M
VIN
2
VS +VS 2 8k
7
CF
3
100k OUTPUT
6
BIAS SECTION rms CORE
4.7 F 9V 4.7 F + 33 F 100k
-VS
4
OUTPUT AMPLIFIER
CAV
5
+ CF (OPTIONAL) 10 F
Figure 7. Battery-Powered Option
CC CC 8k +
1
VIN
AD736
FULL WAVE RECTIFIER INPUT AMPLIFIER
8
COM
VIN
2
7 +VS
3
6
OUTPUT
Figure 8. Low Z, AC-Coupled Input Connection
REV. E
-11-
AD736
OUTLINE DIMENSIONS 8-Lead Standard Small Outline Package [SOIC] Narrow Body (R-8)
Dimensions shown in millimeters and (inches)
5.00 (0.1968) 4.80 (0.1890)
8 5 4
4.00 (0.1574) 3.80 (0.1497)
1
6.20 (0.2440) 5.80 (0.2284)
1.27 (0.0500) BSC 0.25 (0.0098) 0.10 (0.0040) COPLANARITY SEATING 0.10 PLANE
1.75 (0.0688) 1.35 (0.0532) 8 0.25 (0.0098) 0 0.19 (0.0075)
0.50 (0.0196) 0.25 (0.0099)
45
0.51 (0.0201) 0.33 (0.0130)
1.27 (0.0500) 0.41 (0.0160)
COMPLIANT TO JEDEC STANDARDS MS-012AA CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
8-Lead Ceramic Dual In-Line Package [CERDIP] (Q-8)
Dimensions shown in inches and (millimeters)
0.005 (0.13) MIN
8
8-Lead Plastic Dual In-Line Package [PDIP] (N-8)
Dimensions shown in inches and (millimeters)
0.375 (9.53) 0.365 (9.27) 0.355 (9.02)
8 5
0.055 (1.40) MAX
5
PIN 1
1 4
0.310 (7.87) 0.220 (5.59)
1
4
0.295 (7.49) 0.285 (7.24) 0.275 (6.98) 0.325 (8.26) 0.310 (7.87) 0.300 (7.62) 0.015 (0.38) MIN SEATING PLANE 0.060 (1.52) 0.050 (1.27) 0.045 (1.14)
0.100 (2.54) BSC 0.405 (10.29) MAX 0.200 (5.08) MAX 0.200 (5.08) 0.125 (3.18) 0.023 (0.58) 0.014 (0.36) 0.060 (1.52) 0.015 (0.38) 0.150 (3.81) MIN SEATING 0.070 (1.78) PLANE 0.030 (0.76) 15 0 0.015 (0.38) 0.008 (0.20) 0.320 (8.13) 0.290 (7.37)
0.180 (4.57) MAX 0.150 (3.81) 0.130 (3.30) 0.110 (2.79) 0.022 (0.56) 0.018 (0.46) 0.014 (0.36) 0.100 (2.54) BSC
0.150 (3.81) 0.135 (3.43) 0.120 (3.05)
0.015 (0.38) 0.010 (0.25) 0.008 (0.20)
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETERS DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
COMPLIANT TO JEDEC STANDARDS MO-095AA CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
Revision History
Location Page 1 3 4 4 1 3 6 6 8 8 4/03--Data Sheet changed from REV. D to REV. E. Changes to GENERAL DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes to SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes to ABSOLUTE MAXIMUM RATINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes to ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11/02--Data Sheet changed from REV. C to REV. D. Changes to FUNCTIONAL BLOCK DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes to PIN CONFIGURATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Figure 1 Replaced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes to Figure 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes to Application Circuits Figures 4 to 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OUTLINE DIMENSIONS updated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-12-
REV. E
C00834-0-4/03(E)

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